1. Field of the Invention
The present invention relates to vehicle wheels and more particularly, to a method for manufacturing light weight automotive wheels having the appearance and advantages of machined billet-type automotive wheels. Still more particularly, the present invention is directed to an automotive wheel and a method for manufacturing in which the center section is formed by cold drawing and stamping a single aluminum disk to obtain a desired size and cross-sectional profile.
2. Description of the Prior Art
Certain types of two-piece automotive wheels are comprised of a center section mounted within a generally cylindrical rim. The center section further includes a hub having a mounting surface, and is formed using one piece of material such as a steel or aluminum disk. The cylindrical rim is formed using a second piece of material which is compatible with the material comprising the center section. The formed center section is then fixed within the cylindrical rim by, for example, welding, to form the basic wheel. Additional polishing, plating, or other finishing processes are then used to complete the wheel.
As indicated, the center section of the automotive wheel is generally formed using a single piece of material such as aluminum or steel. For example, in a typical prior art method of manufacturing aluminum automotive wheels, a single aluminum disk ("billet") approximately two inches thick and thirteen inches in diameter is machined to form the center section. The machining steps include turning the aluminum billet to obtain the basic center section shape, milling one or more decorative and/or cooling openings in the machined billet, and drilling the holes for the lug bolts. These steps result in substantial material waste which contributes to the finished billet-type automotive wheel being very expensive. Due to their very high cost, billet-type wheels are often used with custom and exotic automobiles.
In another prior art method of manufacturing aluminum automotive wheels, the center section is cast to form the basic shape. The holes for the lug bolts are then drilled and the casting is polished to form the finished center section. Since almost any center section design can be realized by casting, the number of possible wheel designs is much greater with casting than machining. However, while casting provides more freedom of design than machining, cast automotive wheels are not as strong as machined wheels of similar size or weight.
Specifically, since the aluminum used in casting is not pure the microscopic structure of the cast hub is porous, resulting in lower density than a similar sized aluminum billet. In contrast, the aluminum billet is rolled and/or extruded to a given density in order to achieve a desired strength and ductility. Thus, in order to obtain a cast wheel of similar strength as that of a machined wheel, the cast wheel must be of greater mass or have a different shape. Therefore, cast aluminum wheels are typically heavier and bulkier than machined aluminum wheels.
In a prior art method of manufacturing steel automotive wheels, the center section is formed by stamping a steel disk into the desired cross-sectional profile. Since a steel disk of a given thickness is stronger than an aluminum disk of the same thickness, a relatively thin steel disk can be stamped using, for example, a conventional punch press. However, steel automotive wheels are much heavier than aluminum automotive wheels and, further, may be subject to corrosion. In addition, steel automotive wheels are generally considered less stylish than aluminum wheels.
The cross-sectional profile of the center section of aluminum or steel wheels can range from concave (i.e. dish-shaped) to convex (.i.e. crowned). Additionally, the mounting surface of the hub must be offset a specified distance from the inside edge of the rim. Both the cross-sectional profile and the mounting surface offset determine the required thickness of the hub, as measured from the front center surface of the hub to the mounting surface. Thus, in order to obtain a particular profile and offset, machined wheels employ an aluminum billet whose thickness generally corresponds to the required thickness of the hub.